The present invention relates to disk drives such as a hard disk drive (hereinafter referred to as “HDD”), and a control method thereof. In particular, the invention relates to a disk drive capable of performing positioning control of a head by means of the feedforward control and the feedback control, and a control method thereof.
Means for coping with an offtrack error caused by vibrations is indispensable for a data storage device for writing data to a rotating recording medium (hereinafter referred to as “disk drive”), such as a hard disk drive. An example in which the offtracking causes a problem is, for example, a case where while data is written, the offtracking causes data, which is written to a data track other than a target data track, to be rewritten.
In recent years, the track density increases with the increase in capacity of a recording medium. Therefore, an offtrack error tends to easily occur due to vibrations generated when a disk drive itself seeks a head, and also due to vibrations generated by adjacent disk drives in a system such as a disk array. In addition, because the larger capacity and the further speedup are being achieved, the speedup in the servo technique used for moving a head is required. Therefore, if the servo technique for head moving uses only the feedback control that drives a head on the basis of a feedback error between target information used to move the head to a target position and demodulation information into which feedback information from the head is demodulated, the delay of a driving signal becomes so large that the delay cannot be ignored with respect to the seek time. Accordingly, it is impossible to achieve high-speed move. Because of the above reason, the feedforward control is used in general. In the feedforward control, a waveform of an electric current for driving a head is stored beforehand as feedforward information, and the head is controlled with the feedforward information added to a feedback error.
Incidentally, the feedback control is a servo system that feeds back a position error signal (PES) to the positioning control of a head, wherein feedback is applied so that a value of the position error signal becomes the smallest to control positioning of the head. However, if unpredictable random vibrations occur due to an external event, there is a possibility that an offtrack error will occur. For this reason, a device is disclosed in which vibrations influencing a servo system are detected, and thereby the servo system is compensated for. More specifically, the compensation is achieved by: on the basis of a position error signal, detecting a frequency of vibrations that exert an influence upon an HDD; configuring a correction filter that is suitable for the reduction of the vibrations for the detected frequency; and correcting the feedback control by use of this correction filter. See patent document 1 (Japanese Patent Laid-Open No. 2000-048509).
On the other hand, the feedforward control, which is also called an RVS (Rotational Vibration Safeguard) system or an RV-FF (Rotational Vibration Feed Forward) function, is a method in which a head is so controlled that a head moves along the center of a track. This control is performed by, when a sensor detects vibrations, adding a correction signal whose magnitude is proportional to the vibrations, to a servo loop associated with an actuator of a disk drive.
In a hard disk drive, a voice coil motor (VCM) pivotally moves an arm, which supports a head, about a rotating shaft. This causes the head to seek a desired track of a recording medium. Therefore, a relative position between the VCM and the track of the recording medium is changed by vibrations which cause the disk drive to rotate in a direction parallel to a pivot surface of the arm (hereinafter referred to as “vibrations in the rotational direction”), so that offtracking occurs. For this reason, the disk drive is provided with a sensor for detecting vibrations in the rotational direction. There are disclosed a data storage device in which correction information from such an acceleration sensor is added to a servo loop to prevent, on the basis of the output of the sensor, correction operation from being improperly performed, and an offtrack control method thereof. See, e.g., patent document 2 (Japanese Patent Laid-Open No. 2003-346469). According to this method, a signal indicating an error of a head position is inputted to control an electric current to be supplied to a VCM so that the head position is properly corrected by the signal. Moreover, in this method, the input of a correction signal into a servo loop is stopped in the specified timing.